There are numerous individual operations, or processing steps, performed, in a strictly followed sequence, on the silicon wafer in the course of manufacturing a complex integrated circuit (IC). Each such operation must be precisely controlled in order to assure that the entire fabrication process yields integrated circuits displaying the required electrical characteristics.
Frequently, failure of an individual operation is detected only after the completion of the entire, very expensive, process of IC fabrication. Due to the very high cost of advanced IC fabrication processes, such failures result in severe financial losses to the integrated circuit manufacturer. Therefore detection of errors in the manufacturing process, immediately after their occurrence, could prevent the unnecessary continuation of the fabrication of devices which are destined to malfunction, and hence, could substantially reduce the financial losses resulting from such errors.
Process monitoring in semiconductor device manufacturing relies upon the examination of the changes which occur in certain physical and/or chemical properties of the silicon wafer upon which the semiconductor devices are fabricated or in a silicon monitor wafer (i.e. a wafer used for control or a pseudo wafer used for measurement) included within a batch of product wafers. These changes may occur following the various processing steps to which a silicon wafer is subjected and are reflected by changes in the electrical and/or optical properties of the wafer. Therefore, by monitoring selected electrical and/or optical properties of the silicon wafer in the course of IC fabrication, an effective real-time control over the manufacturing process can be accomplished.
Not all of the electrical characteristics of a completed integrated circuit can be predicted based on the measurements performed on a partially processed product or monitor wafer. Most of the characteristics however, can be predicted directly or indirectly based on the investigation of the condition of the front surface of the silicon wafer (substrate) in the course of IC manufacture. The condition of the silicon surface is very sensitive to the outcome of the individual processing steps which are applied during IC manufacturing, and hence, the measurement of the electrical properties of the substrate surface can be an effective tool by which the monitoring of the outcome of the individual processing steps can be accomplished.
Measurement of the electrical and/or optical properties of the substrate typically requires that the substrate be removed from the manufacturing system, measured and then replaced back into the manufacturing system for adjustment. Thus, determination of the electrical characteristics of a substrate resulting from IC fabrication, such as implantation, often results in a delay between the time of implantation and measurement. Providing real-time feedback from the measurement device to the tools employed during IC fabrication would improve processing efficiency and yield rates.